The invention concerns a device to take in fumes and cool the roof of furnaces for melting metals, particularly electric arc furnaces.
The invention is applied mainly in electric arc furnaces employed in steel plants to melt metals, whether they be AC or DC.
The state of the art includes cooled roofs used to cover electric arc furnaces, which have a central aperture to position and move the electrodes, and a peripheral aperture, or fourth hole, to suck fumes and volatile slag from the inside and discharge them from the furnace.
The systems to cool the roofs use pipes structured in panels in which cooling fluid circulates; this prevents the roof from overheating and prevents wear and damage.
One problem in conventional cooling systems is that there is not a uniform distribution of the temperatures which develop on the inner surface of the roof. In fact, the temperatures which develop on the central part, where the electrodes are located, are much higher than those which develop in the peripheral part of the roof.
Moreover, the temperature of the roof in proximity with the aperture to discharge the fumes is much higher than that which develops in the zone diametrically opposite, and it progressively increases as it gets closer to said aperture due to the flow of incandescent fumes conveyed towards this zone. Due to the presence of intake systems connected to the fourth hole, there is a concentrated intake on a limited part of the volume of the furnace, with consequent localized wear and deterioration.
Conventional cooling systems do not always ensure an effective heat protection which will prevent localized wear in those parts most subject to overheating.
Moreover the coefficient of heat flow removal given by such conventional systems is uniform over the whole surface of the roof; consequently a removal coefficient has to be guaranteed over the whole roof which will be at least equal to that required in the zone where the highest temperatures are reached, that is to say, near the fourth hole.
As a consequence, for a large part of the inner surface of the roof, the cooling system is oversized, which implies a high energy consumption and an excessive quantity of cooling fluid, whereas the hottest zones always work at a very high temperature, with the risk of breakages and malfunctions of the cooling pipes.
The conduits wherein the cooling fluid circulates, as made in the state of the art, can have a ring-shaped or helical circular development or a radial development from the center of the roof towards the periphery or vice versa.
However, such conduits have a structure arranged on a single horizontal plane cooperating with the inside of the roof and in particular with the zone surrounding the electrodes; this does not permit a sufficient accumulation of insulating material, such as slag or otherwise, which can assist said panels in their cooling action and heat insulation.
A further problem which conditions the working life of roofs is that the lining which covers the central part of the roof can be damaged by the heat radiated by the electrodes.
The present Applicant, in the patent applications EP-A-805.325 and PCT/IB00/00035, proposed cooling devices for the roof of electric arc furnaces which solve some of the shortcomings explained above.
This invention has been devised, tested and embodied in order to further perfect conventional cooling devices and to obtain other advantages as identified hereafter.
The purpose of the invention is to achieve a device to take in fumes and cool the roof for electric furnaces which will allow to obtain an optimum heat insulation, and hence a better performance of the furnace, with reduced management costs and reduced risks of localized deterioration.
A further purpose is to achieve an intake and cooling device with a much lesser risk of breakages compared with conventional systems, particularly in the central part of the roof which comprises and surrounds the aperture through which the electrodes are introduced, thus allowing to reduce stoppages between one cycle and the other to carry out repairs.
To be more exact, one purpose of the invention is to make possible not to use refractory material in the zone of the roof surrounding the electrodes.
Another purpose is to guarantee a homogeneous and uniform fume intake for the whole volume of the furnace, avoiding those problems deriving from having an intake concentrated in a small zone.
A further purpose is to reduce to a minimum, and even prevent, the possibility that fumes should emerge from the apertures around the electrodes from inside the furnace, or that air should enter the furnace from outside; this allows to increase and make the intake uniform around the electrodes.
The intake and cooling device according to the invention, in a preferential embodiment, substantially consists of three distinct systems which cooperate with each other:
a first cooling system to cool the central zone of the roof, cooperating with the aperture through which the electrodes are introduced,
a second system able to take in and convey the fumes arriving from the first system, and
a third system able to cool the peripheral part comprised between the central zone and the outer perimeter.
The first cooling system, according to a first characteristic of the invention, has cooling pipes arranged in such a manner as to create a cyclone spiral, substantially vertical and ring-shaped, which surrounds and cools the zone around the aperture through which the electrodes are introduced and moved.
According to a variant, this spiral partly surrounds the electrodes.
According to another variant, the cooling pipes are made of a material resistant to high temperatures.
This cyclone spiral allows on the one hand to prevent the fumes from emerging outside from the apertures which surround the electrodes, and on the other hand allows to make the intake action around the electrodes uniform, over the whole circumference of the roof.
Moreover, the spiral conformation of the pipes allows to position the pipes in close proximity to the electrodes.
In a first embodiment of the invention, this first cooling system arranged in the central part of the roof is autonomous with respect to the main fume transport system which is connected to the fourth hole of the furnace and is associated with its own means to take in and discharge the fumes.
In another embodiment, this first cooling system is connected to the main intake and discharge system by means of a connection conduit.
According to a variant, this connection conduit is cooled. According to another variant, this connection conduit includes means to regulate and balance the flow, for example grids or gates, either fixed or movable.
In a first embodiment, the connection conduit is inside the furnace while, according to a variant, it is at least partly outside the furnace.
In a preferential embodiment of the invention, this cyclone spiral has a pitch between the pipes which can vary along its circular development; to be more exact, this pitch is at its minimum, that is to say, the distance between the turns is less, in correspondence with the position of the fume-discharge aperture, and is at its maximum, that is to say, with a greater distance between the turns, in a diametrically opposite position.
This variability of the pitch allows maintaining the intake of the fumes, from inside the furnace towards the outside of the central spiral, as uniform as possible, at a substantially constant value. This allows to make the temperatures of the roof substantially uniform, preventing the zone in proximity with the discharge aperture from being subjected to higher heat loads due to the intense flow of fumes conveyed towards said zone.
To be more exact, the variability in the density of the turns allows to correlate the entity of the cooling action to the higher or lower temperatures which develop in the specific zones of the roof, thus allowing the obtaining of energy savings and in general savings in the management costs of the cooling device. This solution also allows sizing the cooling action of the cooling device better, at the same time keeping a high level of safety and efficiency.
The spiral can be substantially of any shape, provided that it can define at least a collector system to collect and subsequently discharge the fumes with a cyclone development.
According to another characteristic of the invention, on its inner surface exposed to the electrodes, the central spiral has at least one and advantageously two circular pipes arranged on a substantially horizontal plane. The pipes encourage a layer of protective slag to form which melts, in the event of discharges of the electric arc from the electrodes towards the pipes, and creates a protective screen which prevents the pipes from being destroyed.
According to one embodiment of the invention, in cooperation with the inner wall of the cyclone spiral there is at least one cooling system, for example of the type conventionally known as microspray, which acts on the electrode, or on the electrodes, in order to reduce the surface oxidation thereof.
According to another characteristic, the central spiral is covered at the upper part by at least a layer of high-density concentric pipes. The function of the pipes is to protect the spiral from the superheating caused by the irradiance of the heat from the electrodes when they are raised to be removed from the furnace, or simply to be moved between one melting cycle and the next or during the melting cycle itself. According to a variant, the layer of concentric pipes can be replaced, either partly or totally, by a layer of refractory material.
The second system to take in and convey the fumes comprises a series of cooling pipes, arranged coaxial and superimposed so as to create a cooled channel through which the fumes pass, and a cooled conduit of cooling pipes. According to a variant, this conduit is not cooled.
The conduit then connects with the main cooled conduit through which the fumes are conveyed towards the intake and filter systems.
The fumes conveyed through the cyclone spiral can be directed inside this cooled conduit to then be discharged.
According to one embodiment of the invention, at inlet to the main conduit there are means to regulate the flow, for example a movable gate, a grid, fixed or movable, or other suitable means. The function of these means to regulate the flow is to balance the delivery of the flow, facilitating a more uniform and less turbulent discharge.
According to a variant, the fumes conveyed through the cyclone spiral are directed towards an independent discharge conduit, which can be cooled or not.
The third cooling system to cool the peripheral part of the roof has a plurality of radial turns, lying on a substantially vertical plane and with a substantially trapezoid or triangular section. The turns are cooling pipes which define a circular transit channel for the discharge fumes, to convey them towards the discharge aperture.
There are cooling pipes cooperating with the radial turns, and above them; they are arranged on a substantially horizontal plane so as to form a plurality of coils each one covering a defined sector of the periphery of the roof.
According to one feature of the invention, the coil-shaped horizontal pipes are arranged in such a manner that the free intake areas enclosed by each sector are all substantially equal to each other. This causes a uniform movement of the discharge fumes along the global surface of the roof and is achieved by making the pipes in such a manner that the distance between the pipes increases from the periphery to the center of the roof.